1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus for processing a sheet having cutting unevenness by a sheet processing device after forming an image on the sheet.
2. Description of the Related Art
Conventionally, an image forming apparatus such as a copier, a printer, or a facsimile machine includes a sheet conveying device for conveying a sheet to an image forming portion. The sheet conveying device may include a skew feed correcting portion for correcting skew feed of a sheet and displacement thereof in a direction orthogonal to a conveying direction of the sheet (hereinafter, referred to as “width direction”) in order to adjust the posture and position of the sheet by the time the sheet is conveyed to the image forming portion.
In recent years, various sheets such as coated paper, embossed paper, super-thick paper, super-thin paper are used in an image forming apparatus. Therefore, there is a demand for an image forming apparatus which is capable of correcting skew feed at high speed with high accuracy so as to handle any kind of sheets to be used, in addition to having high productivity.
In order to achieve high-speed and high-accuracy skew feed correction, there is a technology in which two skew feed correcting roller pairs are provided at a predetermined interval in a direction orthogonal to a sheet conveying direction, and a skew feed detection sensor for detecting skew feed of the leading edge of a sheet is provided on a downstream side of the skew feed correcting roller pairs in the conveying direction. In such a conventional skew feed correcting portion of an active skew feed correction system, first, the skew feed of a sheet is detected based on a detection signal of the skew feed detection sensor.
Then, the skew feed correcting roller pairs that are driven independently are increased or decreased in speed in accordance with the detected skew feed amount of the sheet. Accordingly, the sheet is turned so that the skew feed of the sheet is corrected. The conventional skew feed correcting portion of an active skew feed correction system corrects skew feed by turning a sheet in accordance with the skew feed amount of the sheet without stopping the sheet. This enables high-speed and high-accuracy skew feed correction for the sheet.
Ideally, the shape of a cut sheet to be used by a user is rectangular. However, the cut sheet is often obtained by cutting a sheet with two blades parallel to each other, and hence, in practice, unevenness occurs during cutting. As the cutting unevenness, cutting unevenness in the shape of a parallelogram as illustrated in FIG. 6A often occurs. In addition, there is cutting unevenness in shape as illustrated in FIGS. 6B and 6C.
Even in sheets having the same size, an angle formed by the leading edge of the sheet and the side edge thereof may not be a right angle as illustrated in FIGS. 6A to 6C. In this case, when skew feed is corrected based on a side B of the leading edge of the sheet, image margin amounts on the back side of an apparatus defining lateral registrations become d41, d42, and d43, respectively, as illustrated in FIGS. 6A to 6C. In the case where image margin amounts are varied, the image position on the sheet is not kept constant.
Therefore, for example, in the case of replacing sheets to be used, a user needs to determine a skew feed adjustment amount by outputting a sampling image once and reading a skew feed amount, and to offset the skew feed correction amount by an amount corresponding to the skew feed adjustment amount. FIG. 7A illustrates the behavior of a sheet S and a resultant product after an image is formed thereon in the case of correcting skew feed of the sheet without inputting a skew feed adjustment amount. FIG. 7B illustrates the behavior of a sheet S and a resultant product after an image is formed thereon in the case of correcting skew feed of the sheet by inputting a skew feed adjustment amount. In general, the skew feed amount of a long side of a sheet is read, and the read skew feed amount is offset to adjust a skew feed amount. However, the user may adjust a skew feed amount based on any one of a short side and a long side of a sheet.
Thus, when the position of an image with respect to a sheet is adjusted, one skew feed adjustment amount (skew feed offset amount) is required for each kind of sheets to be used. There is also cutting unevenness caused by lot differences of sheets, and hence the user needs to reconsider the skew feed adjustment amount every time the lot of the sheets to be used is changed.
Conventionally, there is provided a media library capable of storing the adjustment amounts of skew feed amount and image position set by the user for each kind of sheets and for each container such as a cassette in which sheets are to be contained (Japanese Patent Application Laid-Open No. 2010-089867). In the case where a skew feed amount is adjusted, a skew feed adjustment amount of a sheet stored in the media library is added to a skew feed amount detected by a skew feed detection sensor, to thereby determine a skew feed correction amount. Skew feed is corrected based on the determined skew feed correction amount. Thus, image position accuracy for each sheet is guaranteed.
A conventional image forming apparatus may include a sheet processing device for sequentially performing various processes such as cutting, bookbinding, and punching. When processing a sheet, such a sheet processing device performs the respective processes after aligning the sheet based on the leading edge thereof and the side edge thereof parallel to a sheet conveying direction.
For example, in the case where the sheet processing device aligns a sheet based on the leading edge thereof, a sheet S is pressed by an alignment plate 50 as illustrated in FIG. 8A. At this time, in the case where skew feed is corrected based on the side edge of the sheet and then an image is formed on the sheet as illustrated in FIG. 7B, when the sheet S is pressed by the alignment plate as illustrated in FIG. 8A, the sheet S is skewed. Therefore, a resultant product of the processed sheet S is degraded in quality.
If the shape of a sheet side edge with respect to a sheet leading edge is stored in advance, skew feed can be corrected based on the side edge of the sheet instead of the leading edge thereof. However, if such correction is performed, in the case where the sheet processing device aligns the leading edge of a sheet, the sheet is skewed when pressed by the alignment plate 50 as illustrated in FIG. 8B. Therefore, the reference at a time of formation of an image is still different from the reference of the processing of the processing device, and hence a resultant product of the processed sheet is degraded in quality.
Further, in the case where the sheet processing device aligns the short side of a sheet, a resultant product with high accuracy is obtained in the end if an image is formed based on the short side. However, in a conventional image forming apparatus, the reference of image formation cannot be changed between the short side of a sheet and the long side thereof.